Early Holocene terrestrial climatic variability along a North Atlantic Island transect: palaeoceanographic implications

A synthesis of the early Holocene climatic development in the North Atlantic region is presented, based on three previously published lake records from southern Greenland (Lake N14), Iceland (Lake Torfadalsvatn) and the Faroe Islands (Lake Lykkjuvøtn). The interval 11 500–8500 cal BP has been divided into five phases with respect to the inferred strength of the North Atlantic Current (NAC) and Irminger Currents (IC). Phase 1 (11 500–10 750 cal BP) was characterised by the first establishment of the NAC and IC in the vicinity of the studied sites, interrupted by the Preboreal Oscillation around 11 200 cal BP. Phase 2 (10 750–10 100 cal BP) was marked by a further warming step in southern Greenland rather concordant with a change into colder and more variable winters on the Faroe Islands. It is proposed that this could partly be related to a series of melt water outbursts disturbing the thermohaline circulation in the eastern Atlantic Ocean, resulting in a warming trend in the western region. During Phase 3 (10 100–9400 cal BP) the strength of the IC reaching northwestern Iceland intensified. A more stable regime in surface circulation was established at the onset of Phase 4 (9400–8900 cal BP) in southern Greenland and was followed by a change towards further warm conditions on Iceland at the onset of Phase 5 (8900–8500 cal BP).     

Environmental changes in southeastern Amazonia during the last 25,000 yr revealed from a paleoecological record

New pollen, micro-charcoal, sediment and mineral analyses of a radiocarbon-dated sediment core from the Serra Sul dos Carajás (southeast Amazonia) indicate changes between drier and wetter climatic conditions during the past 25,000 yr, reflected by fire events, expansion of savanna vegetation and no-analog Amazonian forest communities. A cool and dry last glacial maximum (LGM) and late glacial were followed by a wet phase in the early Holocene lasting for ca. 1200 yr, when tropical forest occurred under stable humid conditions. Subsequently, an increasingly warm, seasonal climate established. The onset of seasonality falls within the early Holocene warm period, with possibly longer dry seasons from 10,200 to 3400 cal yr BP, and an explicitly drier phase from 9000 to 3700 cal yr BP. Modern conditions with shorter dry seasons became established after 3400 cal yr BP. Taken together with paleoenvironmental evidence from elsewhere in the Amazon Basin, the observed changes in late Pleistocene and Holocene vegetation in the Serra Sul dos Carajás likely reflect large-scale shifts in precipitation patterns driven by the latitudinal displacement of the Inter-Tropical Convergence Zone and changes in sea-surface temperatures in the tropical Atlantic.     

A 11,000 yr palaeotemperature reconstruction from the southern boreal zone in Finland

The ecotone between the boreo-nemoral (hemiboreal) and the southern boreal vegetation zones constitutes the northern distributional limit of a number of thermophilous tree species in northern Europe and is, to a large extent, controlled by climatic conditions. We present a quantitative annual mean temperature reconstruction from a high-resolution pollen stratigraphy in southern boreal Finland, using a pollen-climate calibration model with a cross-validated prediction error of 0.9°C. Our model reconstructs low but steadily rising annual mean temperature from 10,700 to 9000 cal yr BP. At 8000–4500 cal yr BP reconstructed annual mean temperature reaches a period of highest values (Holocene thermal maximum) with particularly high temperatures (2.0–1.5°C higher than at present) at 8000–5800 cal yr BP. From 4500 cal yr BP to the present-day, reconstructed annual mean temperature gradually decreases by ca 1.5°C. Comparison of present results with palaeotemperature records from the Greenland ice cores, notably with the NorthGRIP δ¹⁸O record, shows marked similarities, suggesting parallel large-scale Holocene temperature trends between the North Atlantic and North European regions. The verification of the occurrence, timing, and nature of the short-term temperature fluctuations during the Holocene in the southern boreal zone in Europe requires replicate, high-resolution climate reconstructions from the region.     

Temperate rainforest response to climate change and disturbance agents in northwestern Patagonia (41°S) over the last 2600 years

We present detailed pollen and charcoal records from Lago Pichilafquén (~ 41°S) to decipher the effects of climate change and varying disturbance regimes on the composition and structure of the vegetation on the Andean foothills of northwestern Patagonia during the last 2600 yr. Here, temperate rainforests have dominated the landscape since 2600 cal yr BP with variations ranging from cool-temperate and wet north Patagonian rainforests to relatively warm and summer-drought-resistant Valdivian rainforests. We interpret relatively warm/dry conditions between 1900–2600, 690–750 and 320–430 cal yr BP, alternating with cold/wet conditions between 1500–1900, 750–1100 and 430–690 cal yr BP. Rapid deforestation and spread of plants introduced by Europeans occurred at 320 and 140 cal yr BP. The record includes five tephras with ages of 2130, 1460, 1310, 1210, and 340 cal yr BP, all of which precede local fire events and increases in trees favored by disturbance by less than 100 yr. We conclude that centennial-scale changes in the southern westerlies were the primary driver of vegetation shifts in northwestern Patagonia over the last 2600 yr. Within this interval, local disturbance regimes altered the structure, composition, and dynamics of the lowland rainforest vegetation during several discrete, short-lived episodes.     

Late Holocene monsoon climate as evidenced by proxy records from a lacustrine sediment sequence in western Guangdong,South China

The study of a 300-cm-thick exposed lacustrine sediment section in the Hedong village in Zhaoqing area which is located in sub-tropical west Guangdong Province in South China, demonstrates that the lacustrine sedimentary sequence possibly contains evidence for exploring variation of Asian monsoon climate. Multi-proxy records, including the humification intensity, total organic carbon, and grain size fractions, reveal a general trend towards dry and cold conditions in the late Holocene that this is because of a decrease in solar insolation on an orbital scale. Three intensified Asian summer monsoon (ASM) intervals (∼3300–3000 cal yr BP, ∼2600–1600 cal yr BP, and ∼900–600 cal yr BP), and three weakened ASM intervals (∼4000–3300 cal yr BP, ∼3000–2600 cal yr BP, and ∼1600–900 cal yr BP) are identified. Our humification record (HD_cal) shows a good correlation on multi-centennial scale with the tree ring Δ¹⁴C record, a proxy of solar activity. A spectral analysis of HD_cal reveals four significant cycles, i.e., ∼1250 yr, 300 yr, 110 yr, and 70 yr, and most of these cycles are related to the solar activity. Our findings indicate that solar output and oceanic–atmospheric circulation probably have influenced the late Holocene climate variability in the study region.     

Chironomid record of Late Quaternary climatic and environmental changes from two sites in Central Asia (Tuva Republic,Russia)—local,regional or global causes?

Sediment cores from two mountain lakes (Lake Grusha at 2413 m a.s.l. and Ak-Khol at 2204 m a.s.l.) situated in the Tuva Republic (southern Siberia, Russia), just north of Mongolia, were studied for chironomid fossils in order to infer post-glacial climatic changes and to investigate responses of the lake ecosystems to these changes. The results show that chironomids are responding both to temperature and to changing lake depth, which is regarded as a sensitive proxy of regional effective moisture. The post-glacial history of this mountain region in Central Asia can be divided into seven successive climatic phases: the progressive warming during the last glacial–interglacial transition (ca 15.8–14.6 cal kyr BP), the warm and moist Bølling-Allerød-like interval (ca 14.6–13.1 cal kyr BP), the cool and dry Younger Dryas-like event (ca 13.1–12.1 cal kyr BP), warmer and wetter conditions during ca 12.1–8.5 cal kyr BP, a warm and dry phase ca 8.5–5.9 cal kyr BP, cold and wet conditions during ca 5.9–1.8 cal kyr BP, as well as cold and dry climate within the last 1800 years. The chironomid records reveal patterns of climatic variability during the Late-glacial and Holocene, which can be correlated with abrupt climatic events in the North Atlantic and the Asian monsoon-dominated regimes. Apparently, the water balance of the studied lakes is controlled by the interrelation between the dominant westerly system and the changing influence of the summer monsoon, as well as the influence of alpine glacier meltwater supply. It is possible that monsoon tracks could have reached the southwest Tuva, resulting in an increase in precipitation at ca 14.6–13.1 and ca 12.1–8.5 cal kyr BP, whereas cyclonic westerlies from the North Atlantic were likely responsible for considerable moisture transport accompanying the global Neoglacial cooling at ca 5.9–1.8 cal kyr BP. These events suggest the changes of the regional pattern of atmospheric circulation, which could be in turn induced by the global climatic shifts. Some discrepancies compared with other reconstructions from Central Asia may be associated with regional (spatial) differences between the changing predominant circulation mechanisms and with local differences in uplift and descent of air masses within the complicated mountain landscape. In this paper, we also discuss the possibilities and perspectives for using chironomids in reconstructions of past temperatures and climate-induced changes in water depth of lakes in Central Asia.     

Holocene climate changes in the central Mediterranean as recorded by lake-level fluctuations at Lake Accesa (Tuscany,Italy)

This paper presents a high-resolution lake-level record for the Holocene at Lake Accesa (Tuscany, north-central Italy) based on a range of sedimentological techniques validated in previous studies, with a chronology derived from 43 radiocarbon dates and four tephra layers. It gives evidence of centennial-scale fluctuations with major highstands at ca 11 500, 11 100, 10 200, 9400, 8200, 7300, 6200, 5700–5200, 4850, 4200, 3400, 2600, 1200 and 400 cal BP. Except for the Early Holocene until ca 10 500 cal BP, this pattern of hydrological changes appears to be in agreement with the regional pattern established for west-central Europe. Correlations with the Preboreal oscillation and the 8.2 ka event as well as with the atmospheric ¹⁴C residual series suggest that lake-level fluctuations developed at Accesa in response to (1) final steps of the deglaciation in the North Atlantic area and (2) variations in solar activity. For the period after 4500 cal BP, correlations with other palaeohydrological records from central Italy indicate that lake-level changes reconstructed at Accesa were mainly driven by climatic variations while anthropic activities and local geomorphological factors only played a secondary role. The Accesa lake-level record also highlights millennial-scale variations with a maximal lowstand at ca 9200–7700 cal BP contemporaneous with Sapropel event 1 in the Mediterranean. It was followed by generally higher lake-level conditions. This appears to be the opposite of that observed in Sicily (southern Italy) where a lake-level maximum developed at ca 9000–8200 cal BP and was followed by a general trend towards aridification. These opposite patterns were interpreted as contrasting hydrological responses to orbitally induced changes in summer insolation. This interpretation has to be tested by further lake-level studies in the central Mediterranean region. Finally, correlations between major lowstands and periods of maximal representation of Quercus ilex point to convergences between climate oscillations and Holocene vegetation history in the Accesa region. However, the maximal representation of Abies during the first half of the Holocene, including a time window where lake level reached a minimal level, suggests a more subtle impact of seasonality processes.     

Burning peat and reworking loess contribute to the formation and evolution of a large Carolina-bay basin

Carolina bays are nearly ubiquitous along ~ 1300 km of the North American Atlantic Coastal Plain, but relatively few bays have been examined in detail, making their formation and evolution a topic of controversy. The Lake Mattamuskeet basin, eastern North Carolina, USA, is a conglomeration of multiple Carolina bays that form a > 162 km² lake. The eastern shoreline of the lake is made up of a 2.9-km-wide plain of parabolic ridges that recorded rapid shoreface progradation. The lower shoreface deposit contains abundant charcoal beds and laminae dated 6465–6863 cal yr BP, corresponding with initiation of a lacustrine environment in the eastern part of the lake. A core from the western part of the lake sampled a 1541–1633 cal yr BP charcoal bed at the base of the lacustrine unit, indicating formation of this part of the basin postdates the eastern basin. Lake Mattamuskeet has no relationship to the Younger Dryas or a linked impact event because rim accretion significantly postdates 12,000 cal yr BP. The shoreline progradation, and association of charcoal beds with the oldest lake sediment in both main parts of the basin, suggest that fire and subsequent hydrodynamic processes were associated with initial formation of these Carolina bays.     

Interactions between climate and vegetation during the Lateglacial period as recorded by lake and mire sediment archives in Northern Italy and Southern Switzerland

We reconstruct the vegetational history of the southern side of the Alps at 18,000–10,000 cal yr BP using previous and new AMS-dated stratigraphic records of pollen, stomata, and macrofossils. To address potential effects of climatic change on vegetation, we compare our results with independent paleoclimatic series (e.g. isotope and chironomid records from the Alps and the Alpine forelands). The period before 16,000 cal yr BP is documented only at the lowland sites. The previous studies used for comparison with our new Palughetto record, however, shows that Alpine deglaciation must have started before 18,000–17,500 cal yr BP south of the Alps and that deglaciated sites were colonized by open woods and shrublands (Juniperus, tree Betula, Larix, Pinus cembra) at ca 17,500 cal yr BP. The vegetational history of a new site (Palughetto, 1040 m a.s.l.) is consistent with that of previous investigations in the study region. Our results show three conspicuous vegetational shifts delimited by statistically significant pollen zones, at ca 14,800–14,400, 13,300–12,800 and 11,600–11,200 cal yr BP. At sites situated above 1000 m a.s.l. (e.g. Palughetto, Pian di Gembro) forests expanded in alpine environments at ca 14,500 cal yr BP (onset of Bølling period, GI-1 in the Greenland ice record). At the same time, rather closed treeline communities of the lowlands were replaced by dense stands of Pinus sylvestris and Betula. These early forests and shrublands consisted of Larix, P. cembra, Juniperus, P. sylvestris, Pinus mugo, and Betula, and had become established at ca 16,000 cal yr BP, probably in response to a temperature increase. If combined with other records from the Southern Alps, our data suggest that treeline ascended by ca 800–1000 m in a few centuries at most, probably as a consequence of climatic warming at the beginning of the Bølling period. At 13,100–12,800 cal yr BP the onset of a long-lasting decline of P. sylvestris was accompanied by the expansion of Quercus and other thermophilous tree taxa below ca 600 m a.s.l. This vegetational change was probably induced by a shift to warmer climatic conditions before the onset of the Younger Dryas, as indicated by independent paleoclimatic records. Only a few centuries later, at ca 12,700–12,500 cal yr BP, an expansion of herbaceous taxa occurred in the lowlands as well as at higher altitudes, documenting an opening of forested habitats. This change coincided with the beginning of the Younger Dryas cooling (GS-1), which according to the paleoclimatic series (e.g. oxygen isotope series), started at 12,700–12,600 cal yr BP and lasted for about 1000 years. Environments south of the Alps responded markedly to climatic warming at the onset of the Holocene (11,600–11,500 cal yr BP). Thermophilous trees that had declined during the Younger Dryas re-expanded very rapidly in the lowlands and reached the high altitude sites below ca 1500 m a.s.l. within a few centuries at most. Our study implies that the synchronous vegetational changes observed over wide areas were probably a consequence of abrupt climatic shifts at the end of the Last Glacial Maximum (LGM) and during the Lateglacial. We emphasize that important vegetational changes such as the expansion of forests occurred millennia before the onset of similar processes in northwestern and central Europe.     

Holocene palaeohydrological changes in the northern Mediterranean borderlands as reflected by the lake-level record of Lake Ledro,northeastern Italy

A lake-level record of Lake Ledro (northern Italy) spans the entire Holocene with a chronology derived from 51 radiocarbon dates. It is based on a specific sedimentological approach that combines data from five sediment profiles sampled in distinct locations in the littoral zone. On a millennial scale, the lake-level record shows two successive periods from 11,700 to 4500 cal yr BP and from 4500 cal yr BP to the present, characterized by lower and higher average lake levels, respectively. In addition to key seasonal and inter-hemispherical changes in insolation, the major hydrological change around 4500 cal yr BP may be related to a non-linear response of the climate system to orbitally-driven gradual decrease in insolation. The Ledro record questions the notion of an accentuated summer rain regime in the northern Mediterranean borderlands during the boreal insolation maximum. Moreover, the Ledro record highlights that the Holocene was punctuated by successive centennial-scale highstands. Correlations with the Preboreal oscillation and the 8.2 ka event, and comparison with the atmospheric ¹⁴C residual record, suggest that short-lived lake-level fluctuations developed at Ledro in response to (1) final steps of the deglaciation in the North Atlantic area and (2) variations in solar activity.     

Holocene vegetation and climate change recorded in alpine bog sediments from the Borreguiles de la Virgen,Sierra Nevada,southern Spain

High-resolution pollen and magnetic susceptibility (MS) analyses have been carried out on a sediment core taken from a high-elevation alpine bog area located in Sierra Nevada, southern Spain. The earliest part of the record, from 8200 to about 7000 cal yr BP, is characterized by the highest abundance of arboreal pollen and Pediastrum, indicating the warmest and wettest conditions in the area at that time. The pollen record shows a progressive aridification since 7000 cal yr BP that occurred in two steps, first shown by a decrease in Pinus, replaced by Poaceae from 7000 to 4600 cal yr BP and then by Cyperaceae, Artemisia and Amaranthaceae from 4600 to 1200 cal yr BP. Pediastrum also decreased progressively and totally disappeared at ca. 3000 yr ago. The progressive aridification is punctuated by periodically enhanced drought at ca. 6500, 5200 and 4000 cal yr BP that coincide in timing and duration with well-known dry events in the Mediterranean and other areas. Since 1200 cal yr BP, several changes are observed in the vegetation that probably indicate the high-impact of humans in the Sierra Nevada, with pasturing leading to nutrient enrichment and eutrophication of the bog, Pinus reforestation and Olea cultivation at lower elevations.     

Holocene climate variability in Europe: Evidence from δ18O,textural and extension-rate variations in three speleothems

Time-series O isotope profiles for three U–Th dated stalagmites have revealed that for much of the Holocene, a site on the Atlantic seaboard (SW Ireland) exhibits first-order δ¹⁸O trends that are almost exactly out of phase with coupled δ¹⁸O curves from two southern European sites (SE France and NW Italy). In the Irish stalagmite (CC3 from Crag Cave, SW Ireland), low δ¹⁸O at 10,000 cal yr BP reflects cool conditions. By the early to mid-Holocene (9000–6000 cal yr BP) δ¹⁸O had increased, reflecting the onset of warmer conditions on the Atlantic seaboard. This shift to higher δ¹⁸O was accompanied by a marked increase in the stalagmite extension rate, reinforcing our interpretation that this was a period of relative warmth. Except for an episode of increased extension rate about 5500 yr ago, δ¹⁸O in the Crag stalagmite exhibits a gradual decrease, accompanied by declining extension rates between 7800 and 3500 cal yr BP, interpreted as a cooling trend. There is evidence for increases in both δ¹⁸O and stalagmite extension rate in the period from 3500 cal yr BP to the present day suggesting a return to warmer conditions on the Atlantic seaboard. In the stalagmite from NW Italy (ER76, Grotta di Ernesto, Trentino province) the early-Holocene (c. 9200-7800 cal yr BP) is characterised by high δ¹⁸O, probably indicative of warm and/or dry conditions. Exceptionally low δ¹⁸O from 7800 to 6900 cal yr BP at this site reflects a well-defined wet phase (Cerin wet phase). In the last three millennia, this stalagmite exhibits a shift to lower δ¹⁸O, interpreted as some combination of cooler and/or wetter conditions. Unlike the Irish stalagmite, the Italian sample does not show a correlation between δ¹⁸O and extension rate. Instead, its extension rate correlates roughly with δ¹³C, presumably reflecting a climate-driven vegetation change. In the early Holocene, δ¹⁸O in the French stalagmite (CL26, Grotte de Clamouse, Herault province, SE France) was low relative to its Holocene average. For much of the period since c. 3500 cal yr BP this stalagmite exhibits higher δ¹⁸O than in the early Holocene, suggesting warmer conditions. Like the Irish stalagmite, the French sample exhibits a well-defined correlation between δ¹⁸O and extension rate. Had drip-water availability been the dominant control on δ¹⁸O at this semi-arid site then higher δ¹⁸O would have been accompanied by lower, not higher extension rates. This suggests strongly that temperature rather than rainfall amount was the dominant control at this site. While conclusions regarding the patterns of climate variability on a continent scale must remain tentative because of the limited number of stalagmites studied we argue that early Holocene warm conditions on the Atlantic seaboard (Irish site) coincided with relatively cool conditions at the Clamouse site. By c. 3500 yr ago the pattern appears to have been reversed.     

Early-Holocene climatic oscillations recorded by lake-level fluctuations in west-central Europe and in central Italy

On the basis of lake-level data, the period of the Preboreal oscillation (PBO) at 11 300–11 150 cal yr BP as defined by the GRIP oxygen-isotope record appears to correspond to wetter climatic conditions in west-central Europe and to a marked drying in north-central Italy. Additional short-lived phases of higher lake level have been identified before the PBO at ca 11 450–11 400 and 11 350 cal yr BP in west-central Europe, and at ca 11 500 cal yr BP in Italy. Such multiple climatic oscillations around the PBO have been observed in various records in northwestern Europe. On the basis of various proxies, a map of changes in moisture over western Europe during the PBO indicates a mid-latitude zone between 58 and 43°N characterised by wetter conditions, while a drier climate developed in southern and northern Europe. This wet middle zone shows a larger extension and suggests a more meandering, weaker Atlantic Westerly Jet during the PBO than during the 8.2 kyr event. A comparison of lake-level records in west-central Europe with (1) outbursts from North American and north-European proglacial lakes, and (2) variations in solar activity as reflected by ¹⁴C and the ¹⁰Be records supports the hypothesis that the PBO was a response to successive meltwater pulses at 11 300, 11 250, 11 200 and 11 170 cal yr BP and to a sudden decrease in solar activity at 11 250 cal yr BP. This study points to the necessity of developing integrated multi-proxy approaches to construct more robust regional event stratigraphies, and of better documenting palaeohydrological changes in the Mediterranean area.     

Abrupt climate warming in East Antarctica during the early Holocene

We report a centennial-scale warming event between 8600 and 8400 cal BP from Amery Oasis, East Antarctica, that is documented by the geochemical record in a lacustrine sediment sequence. The organic carbon content, the C/S ratio, and the sedimentation rate in this core have distinctly elevated values around 8500 y ago reflecting relatively warm and ice-free conditions that led to well-ventilated conditions in the lake and considerable sedimentation of both autochthonous and allochthonous organic matter on the lake bottom. This abrupt warming event occurred concurrently with reported warm climatic conditions in the Southern Ocean while the climate in central East Antarctic remained cold. The comparison of the spatial and temporal variability of warm climatic periods documented in various terrestrial, marine, and glacial archives from East Antarctica elucidates the uniqueness of the centennial-scale warming event in the Amery Oasis. We also discuss a possible correlation of the Amery warming event with the abrupt climatic deterioration around 8200 cal BP on the Northern Hemisphere.     

7000 years of paleostorm activity in the NW Mediterranean Sea in response to Holocene climate events

A high-resolution record of paleostorm events along the French Mediterranean coast over the past 7000 years was established from a lagoonal sediment core in the Gulf of Lions. Integrating grain size, faunal analysis, clay mineralogy and geochemistry data with a chronology derived from radiocarbon dating, we recorded seven periods of increased storm activity at 6300–6100, 5650–5400, 4400–4050, 3650–3200, 2800–2600, 1950–1400 and 400–50 cal yr BP (in the Little Ice Age). In contrast, our results show that the Medieval Climate Anomaly (1150–650 cal yr BP) was characterised by low storm activity.The evidence for high storm activity in the NW Mediterranean Sea is in agreement with the changes in coastal hydrodynamics observed over the Eastern North Atlantic and seems to correspond to Holocene cooling in the North Atlantic. Periods of low SSTs there may have led to a stronger meridional temperature gradient and a southward migration of the westerlies. We hypothesise that the increase in storm activity during Holocene cold events over the North Atlantic and Mediterranean regions was probably due to an increase in the thermal gradient that led to an enhanced lower tropospheric baroclinicity over a large Central Atlantic–European domain.     

Advection of Atlantic Water to the western and northern Svalbard shelf since 17,500 cal yr BP

The changes in flow and character of the warm Atlantic Water through the last 17,500 cal yr are reconstructed from the distribution of benthic foraminifera species, planktonic and benthic foraminifera abundances, stable oxygen isotopes and lithology in two cores from the western and northern shelf of Svalbard. The results show almost continuous presence of Atlantic Water at the shelf areas since >14,500 cal yr BP. The Bølling and Allerød intervals stand out as periods of highest bottom waters temperatures. The strong inflow of saline, but chilled Atlantic Water during the early Holocene was followed by cooling and freshening of the bottom waters during the mid- and late Holocene. The two records reveal synchronous oceanographic changes that are closely tied to changes in the flow of Atlantic Water recorded further south in the Nordic seas. The early Holocene warming was not just an effect of higher solar insolation, but was also due to increased heat flux from the stronger Atlantic Water inflow driven by wind force and/or thermohaline circulation.     

Holocene vegetation and climatic variations in Central India: A study based on multiproxy evidences

Palynology, texture, mineralogy, geochemistry, and magnetic susceptibility analysis of a 2 m deep sediment core from Padauna Swamp, southeastern Madhya Pradesh infers that between 8600 and 7500 cal yr BP a warm and relatively less-humid climate prevailed with open tree-savannahs dominated by grasses followed by sedges, Artemisia and members of Chenopodiaceae/Amaranthaceae with scanty trees viz., Schrebera, Aegle marmelos and Sterculia urens. This is well supported by lower organic to carbonate carbon ratio, coarser texture having relatively low CIA and magnetic susceptibility values and presence of some primary minerals. Between 7500 and 6250 cal yr BP the tree-savannahs were succeeded by open mixed deciduous forests with the invasion of a few more trees viz., Madhuca indica, Holoptelea, Emblica officinalis, Mitragyna parvifolia and members of Anacardiaceae in response to onset of a warm and humid climate. A considerable rise in organic carbon generated from the degradation of plentiful biomass along with increase in clay content with signs of kaolinite and increase in immobile over mobile elements with slightly higher CIA and magnetic susceptibility values also suggest climatic amelioration. The presence of ruderal plants such as Artemisia, Cannabis sativa and Cheno/Am further infers initiation of human activities in the region. Between 6250 and 2800 cal yr BP, the mixed deciduous forests became more diverse and dense, subduing grasses and other herbaceous elements. Sporadic incursion of Shorea robusta (Sal) in forest floristic was recorded around 5000 cal yr BP. The overall change in the vegetation mosaic reflects that a warm and more-humid climate prevailed in the region, probably on account of invigoration of southwest monsoon. This observation is further corroborated by other proxy data showing a spurt in organic/inorganic carbon ratio, increase in clay content with matured mineralogy, significantly higher CIA and magnetic susceptibility values. Since 2800 cal yr BP onwards, the modern Sal dominated deciduous forests were established indicating continuation of warm and more-humid climate including timely arrival of SW monsoon coinciding with the shedding of Sal seeds as they are viable for a very short period.     

Stable isotope composition of subfossil Cerastoderma glaucum shells from the Szczecin Bay brackish deposits and its palaeogeographical implications (South Baltic Coast,Poland)

The environmental conditions of the Szczecin Bay, which existed prior to Szczecin Lagoon, have been reconstructed on the basis of the stable carbon and oxygen isotope (¹⁸O and ¹³C) analysis and radiocarbon dates obtained for subfossil shells of Cerastoderma (Cardium) glaucum. The shells in the collected core were well preserved in their life positions, representing a geochemical record of past temperature variation over the middle Holocene. Three major periods with different thermal conditions have been distinguished in the interval ~ 6000–4300 cal yr BP, when the important Littorina regional transgression took place. During the first period, 6000–5250 cal yr BP, water temperature decreased by 1.4°C, and then remained constant over the second period (5250–4750 cal yr BP). In contrast, during the third period (4750–4300 cal yr BP) both δ-values were highly variable and the mean summer temperature (March–November) increased by about 3.5°C. During first two periods, δ¹⁸O and δ¹³C were significantly correlated, indicating stability of the environmental conditions.     

A new approach for reconstructing glacier variability based on lake sediments recording input from more than one glacier

We explore the possibility of building a continuous glacier reconstruction by analyzing the integrated sedimentary response of a large (440 km²) glacierized catchment in western Norway, as recorded in the downstream lake Nerfloen (N61°56’, E6°52’). A multi-proxy numerical analysis demonstrates that it is possible to distinguish a glacier component in the ~ 8000-yr-long record, based on distinct changes in grain size, geochemistry, and magnetic composition. Principal Component Analysis (PCA) reveals a strong common signal in the 15 investigated sedimentary parameters, with the first principal component explaining 77% of the total variability. This signal is interpreted to reflect glacier activity in the upstream catchment, an interpretation that is independently tested through a mineral magnetic provenance analysis of catchment samples. Minimum glacier input is indicated between 6700–5700 cal yr BP, probably reflecting a situation when most glaciers in the catchment had melted away, whereas the highest glacier activity is observed around 600 and 200 cal yr BP. During the local Neoglacial interval (~ 4200 cal yr BP until present), five individual periods of significantly reduced glacier extent are identified at ~ 3400, 3000–2700, 2100–2000, 1700–1500, and ~ 900 cal yr BP.     

Synchronous climatic change inferred from diatom records in four western Montana lakes in the U.S. Rocky Mountains

Late-Holocene environmental and climatic conditions were reconstructed from diatom assemblages in sediment cores from four western Montana lakes: Crevice Lake, Foy Lake, Morrison Lake, and Reservoir Lake. The lakes show synchroneity in timing of shifts in diatom community structure, but the nature of these changes differs among the lakes. Two of the sites provide highly resolved records of hydrologic balance, while the other two stratigraphic sequences primarily record temperature impact on lake thermal structure. All four lakes show significant change in five discrete intervals: 2200–2100, 1700–1600, 1350–1200, 800–600, and 250 cal yr BP. The similarities in the timing of change suggest overlying regional climatic influences on lake dynamics. The 800–600 cal yr BP shift is evident in other paleorecords throughout the Great Plains and western US, associated with the transition from the Medieval Climate Anomaly to the Little Ice Age. Large-scale climatic mechanisms that influence these lake environments may result from atmospheric circulation patterns that are driven by interactions between Pacific and Atlantic sea-surface temperatures, which are then locally modified by topography.